1. Field of the Invention
The present invention relates generally to wavelength division multiplexing (WDM) using a plurality of optical signals having different wavelengths, and more particularly to an optical amplifier applied to WDM and an optical transmission system including the optical amplifier.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for amplifying signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pump light source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band, a gain band including a wavelength of 1.55 .mu.m can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
Accordingly, by combining an optical amplifier and WDM, the span and capacity of an optical transmission system can be increased.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by gain characteristics (wavelength dependence of gain) which is represented by a gain deviation or gain tilt. For example, in an EDFA, a gain deviation is produced at wavelengths in the vicinity of 1.55 .mu.m. When a plurality of EDFAs are cascaded to cause accumulation of gain deviations, an optical SNR (signal-to-noise ratio) in a channel included in a band giving a small gain is degraded.
Automatic gain control (AGC) is effective for maintaining the gain characteristics of an optical amplifier constant. However, in the case of performing AGC only, there is a possibility that an optical output level per channel may not be maintained constant. If the optical output level exceeds a tolerable range, transmission characteristics are degraded by the influence of nonlinear effects occurring in an optical fiber transmission line especially in the case of high-speed transmission at rates over 10 Gb/s. Accordingly, to suppress the degradation in transmission characteristics due to the influence of nonlinear effects, stabilization of the optical output level is essential. To this end, automatic output level control (ALC) is effective. However, in the case of performing ALC only, the gain characteristics of an optical amplifier are not always maintained constant.